Does a Freeze Kill Pollen?

Sometimes, but it takes more than one cold night. A hard freeze near 28°F for a few hours kills ragweed and the other fall weeds, and that ends their pollen for the year. A light frost does not.

And a freeze does nothing to the trees that pollinate in winter. In warm regions, the killing freeze never comes at all. Here is what cold actually does to pollen, and when it gives you a break.

What a Hard Freeze Actually Does

Ragweed is an annual plant. It sprouts in spring, pumps out pollen in late summer and fall, then dies when winter arrives. The thing that kills it is a hard freeze: a few hours at about 28°F. Once that happens, the plant stops producing, and ragweed pollen, which affects an estimated 23 million Americans, is done for the season.

A light frost around 32°F is a different story. It nips the tender growth but leaves the hardy pollen producers standing. For the exact end dates by region, see our guide on when ragweed season ends.

One Frost Is Rarely Enough

The first cold night of fall gets a lot of hope pinned on it, and it usually disappoints. A single light frost does not kill the plants. If a warm spell follows, and it often does, ragweed keeps releasing pollen and your symptoms come right back. The season is truly over only after a sustained hard freeze holds. So do not put the tissues away after one chilly morning.

Trees Don't Play by These Rules

Weeds die in the cold. Trees wait it out and pollinate anyway. Mountain cedar, also called juniper, peaks December through February in central Texas, right through freezing weather. That is what drives cedar fever, the Texas winter allergy. Elsewhere, alder and other early trees can start releasing pollen in late winter, weeks before the last frost. Cold air does not stop them.

Warm Regions Never Get the Off Switch

A freeze can only help if it actually happens. On the Gulf Coast, in Florida, and across the desert Southwest, the first hard frost comes late or not at all. So pollen keeps flowing. Some southern and southwestern regions see pollen nearly 12 months of the year.

If you live in Miami, FL or Phoenix, AZ, waiting for a freeze to end your allergies is waiting for something that may never come. Northern cities like Minneapolis, MN get their killing freeze on schedule most years.

Warmer Falls Are Pushing the Freeze Later

The off switch is also arriving later than it used to. Research in the Proceedings of the National Academy of Sciences found North American pollen seasons have grown about 20 days longer since 1990, with the first frost slipping later into the fall. That adds two to three weeks of ragweed to many northern cities compared with a generation ago. A late freeze means a longer sneezing season.

What Cold Does to Pollen

Cold ends weed pollen. It does little for tree pollen, and nothing where it never freezes.

How to Handle the Freeze Transition

• Don't assume one frost ended it. Check your local pollen count before you stop your fall routine. A warm week can bring ragweed back.
• Suspect cedar or juniper if winter symptoms hit. In Texas and the Southwest, freezing weather and high pollen go together. Our cedar fever guide explains the season.
• Look indoors when the air outside is cold and clean. Winter symptoms often trace to dust mites and mold, not pollen. Our guide on humidity and allergies covers the indoor triggers.
• Keep taking your medication until counts actually drop. The calendar does not end pollen season. The freeze, or your local data, does.

If your symptoms run long past your area's first frost every year, it is worth seeing an allergist to pin down the trigger.

Frequently Asked Questions

Does frost kill pollen?

A hard freeze does, for the plants it kills. Around 28°F for several hours ends ragweed and other annual weeds for the year. A light frost near 32°F only slows them, and counts can rebound on the warm days that follow.

What temperature kills ragweed?

About 28°F held for a few hours. A single light frost around 32°F is not enough, which is why one cold night rarely stops your fall symptoms.

Is there pollen in winter?

Yes, in many places. Mountain cedar peaks December through February in central Texas, and warm regions that rarely freeze can have pollen in the air nearly year-round.

Sources

• American College of Allergy, Asthma & Immunology (ACAAI). "Ragweed Allergy." acaai.org
• American College of Allergy, Asthma & Immunology (ACAAI). "Tree Allergy." acaai.org
• Asthma and Allergy Foundation of America (AAFA). "Seasonal Allergies (Rhinitis)." aafa.org
• Anderegg WRL, et al. "Anthropogenic climate change is worsening North American pollen seasons." PNAS. 2021. doi:10.1073/pnas.2013284118

Has pollen actually dropped in your city?

Don't guess from the thermometer. Check today's tree, grass, and weed pollen for your city.